1. Field of the Invention
The present invention relates to a magnetic memory.
2. Related Background Art
Currently, volatile memories such as DRAMs and SRAMs are used as general-purpose memories in information processing equipment such as computers and communication devices. With the volatile memories such as DRAMs, however, it is necessary to constantly supply electric current, e.g., to perform refresh for maintaining stored information, and the whole information will be lost if the power supply is shut off. For this reason, it becomes necessary to provide a means for storing the information, i.e., to provide an additional nonvolatile memory, e.g., a flash EEPROM or a hard disk drive used currently. An important subject for these nonvolatile memories is to increase speed of access with increase in speed of information processing.
These nonvolatile memories, however, are not yet quite satisfactory in terms of access speed, reliability, power consumption, and so on.
Furthermore, rapid spread and enhancement of performance of portable information equipment induced rapid development of information equipment aimed at so-called ubiquitous computing, which permits information processing anytime and anywhere. There are strong demands for development of highly-reliable, high-speed, large-capacity nonvolatile memories as key devices in development of such equipment.
A promising technology effective to increase in speed of the nonvolatile memory is an MRAM Magnetic Random Access Memory) in which magnetic thin-film elements for storing information by directions of magnetizations along an axis of easy magnetization of a ferromagnetic layer are arrayed in a matrix. In the MRAM, information is stored based on directions of magnetizations of two ferromagnets. A magnetization reversing speed of a fine ferromagnet is said to be 2 nsec or less, and thus the MRAM can be a high-speed memory. For reading stored information, a direction of magnetization of a magnetosensitive layer becomes parallel or antiparallel to a direction of reference magnetization to cause a resistance difference and it is detected as a change of electric current or voltage.
The MRAMs include those utilizing the Giant Magnetoresistance (GMR) effect. One of the known MRAMs utilizing the GMR effect is the one described in U.S. Pat. No. 5,343,422. The GMR effect is a phenomenon in which the resistance is minimum when magnetization directions of two magnetic layers parallel to the axis of easy magnetization are parallel and in which the resistance is maximum when the magnetization directions of the two magnetic layers are antiparallel. The MRAMs utilizing the GMR effect include those of a Pseudo spin valve type to write/read information by making use of a difference between retentive forces of two ferromagnets, and those of a Spin Valve type including a fixed layer in which a magnetization direction thereof is fixed by antiferromagnetic coupling to an antiferromagnetic layer with a nonmagnetic layer in between, and a free layer in which a magnetization direction thereof varies depending upon an external magnetic field.
In the MRAMs utilizing the GMR effect, a change in resistance is read as a change of electric current or voltage. In either case, information is written by a method of reversing the magnetization direction of the magnetic layer by an induced magnetic field (current magnetic field) by electric current flowing through wiring.
For further improvement in the resistance change in the GMR, there are proposals on MRAMs utilizing the Tunnel Magnetoresistance (TMR) effect. The TMR effect is a phenomenon in which a tunnel current flowing through an insulating layer varies depending upon a relative angle between magnetization directions of two ferromagnetic layers placed with the thin insulating layer in between. The resistance is minimum when the magnetization directions are parallel; the resistance is maximum when they are antiparallel. In the TMR, for example, CoFe/Al oxide/CoFe demonstrates a large resistance change rate of 40% or more, and high resistance, and thus permits easy impedance matching in combination with semiconductor devices such as MOS-FETs. For this reason, the TMR permits easier achievement of high output than the GMR, and is expected to achieve increase in storage capacity and access speed. The MRAMs utilizing the TMR effect are described in U.S. Pat. No. 5,629,922 and Japanese Patent Application Laid-Open No. 9-91949.
The MRAMs utilizing the TMR effect adopt a method of storing information by changing a direction of magnetization of a magnetic film to a predetermined direction by a current magnetic field of wiring. A method for reading stored information is a method of reading information by letting an electric current flow perpendicularly to an insulating layer and detecting a change in resistance of a thin-film magnetic element.
Many MRAMs have a structure in which TMR elements are located at intersections between bit lines and word lines routed in a lattice pattern. A normal TMR element has a three-layer structure of ferromagnetic layer/nonmagnetic insulating layer/ferromagnetic layer having a nonmagnetic layer between two ferromagnetic layers. The ferromagnetic layers are normally comprised of a transition metal magnetic element (Fe, Co, Ni) or an alloy of transition metal magnetic elements (CoFe, CoFeNi, NiFe, etc.) in the thickness of 10 nm or less, and the nonmagnetic insulating layer is comprised of Al2O3, MgO, or the like.
The direction of magnetization is fixed in one ferromagnetic layer (fixed layer) forming the TMR element, and the direction of magnetization rotates according to the external magnetic field in the other ferromagnetic layer (magnetosensitive layer or free layer). A structure of the fixed layer frequently used is the exchange coupling type in which an antiferromagnetic layer (FeMn, IrMn, PtMn, NiMn, or the like) is given to the one ferromagnetic layer.
Memory information “1” or “0” is defined according to a state of directions of magnetizations of the two ferromagnetics forming the TMR element, i.e., depending upon whether the directions of magnetizations are parallel or antiparallel. The value of electric resistance in the thickness direction is larger in an antiparallel state of the magnetization directions of the two ferromagnetics than in a parallel state of the magnetization directions.
Therefore, the information “1” or “0” is read by letting an electric current flow in the thickness direction of the TMR element and measuring a resistance or an electric current value of the TMR element by MR (magnetoresistance) effect.
The conventional method of writing the information “1” or “0” is to rotate the direction of magnetization of the magnetosensitive layer in the TMR element by action of magnetic fields created by flow of electric current through lines located near the TMR element.
In a case where elements are highly integrated to realize a high-density memory, the magnetoresistive elements are micronized to reduce a ratio of length and thickness of the magnetic layers, and this increases a demagnetizing field and results in increasing the intensity of the magnetic field for changing the magnetization direction of the magnet and requiring a large writing current.
Known technologies for reducing the wiring current include a magnetization reversing method of applying a magnetic field to the magnet in a writing operation of changing the magnetization direction of the magnetosensitive layer corresponding to the information “1” or “0,” and spin injection magnetization reversal using spin transfer torque by spin polarized current.
A general reading method of information is a method of providing each cell with a read select transistor, bringing only the read transistor of a selected cell into a conduction state, and reading a resistance of the magnetoresistive element of the selected cell.
The spin transfer torque is a torque that changes the magnetization direction of the other ferromagnet when an electric current is allowed to flow from one ferromagnet through the nonmagnetic layer to the other ferromagnet. By controlling a spin direction of the injected current, therefore, it becomes feasible to change the magnetization direction of the other magnet.
For example, when an electric current is allowed to flow in a direction perpendicular to a film surface of a laminate consisting of microscopic ferromagnetic layer/nonmagnetic layer/ferromagnetic layer, reversal of magnetization of the ferromagnet takes place. This phenomenon is called spin injection magnetization reversal, and occurs as follows: there is a difference between energy states of electrons with upward spins (up spins) and electrons with downward spins (down spins) at the junction between the ferromagnetic layer and the nonmagnetic layer and this difference causes differences of transmittance and reflectance of up-spin and down-spin electrons, resulting in flow of a spin polarized current.
Spin-polarized electrons of the spin polarized current flowing into the ferromagnetic layer exchange-interact with electrons in the ferromagnetic layer to generate a torque between the electrons, which causes magnetization reversal. This is the magnetization reversal induced by the electric current inside the magnet, different from magnetization reversal induced by the open current magnetic field; therefore, there is little influence on adjacent cells, the writing current is unlikely to increase with micronization of elements, and, conversely, the writing current can be reduced with micronization of elements. When the spin injection magnetization reversal is used as a method of recording information, a high-density magnetic memory can be realized accordingly.
The known methods of changing the direction of magnetization of the ferromagnet by making use of the spin transfer torque include (I) Relaxing Switching method, (II) Precessional Switching method, (III) Relaxing-Precessional Switching method, and so on.
In the relaxing switching method, the direction of magnetization of the magnetosensitive layer is controlled by the spin transfer torque from the fixed layer, and the direction of magnetization of the fixed layer is within the film surface and is parallel to the axis of easy magnetization of the magnetosensitive layer. For reversing the direction of magnetization of the magnetosensitive layer, therefore, the spin transfer torque competes with Spin Relaxing acting to direct the magnetization into the effective magnetic field direction, in an initial stage of reversal. Since the spin transfer torque is small in the initial stage of reversal where the direction of magnetization of the magnetosensitive layer is nearly parallel to the direction of magnetization of the fixed layer, the reversal takes some time. Namely, in the relaxing switching method, the direction of magnetization is gradually changed into an equilibrium state against these forces, and a large electric current is thus needed in order to reverse the direction of magnetization. The magnitude of the spin transfer torque necessary for the magnetization reversal is proportional to the Gilbert attenuation constant in the LLG (Landau-Lifshitz-Gilbert) equation.
In the precessional switching method, the direction of magnetization of the magnetosensitive layer is controlled by the spin transfer torque from the fixed layer, and the direction of magnetization of the fixed layer is perpendicular to the film surface and perpendicular to the axis of easy magnetization of the magnetosensitive layer. The spin transfer torque causes the direction of magnetization of the magnetosensitive layer to have a perpendicular component to the film surface and the demagnetizing field causes the magnetization to rotate into another direction within the film surface. Since the spin transfer torque is constant even after rotation of the magnetization of the magnetosensitive layer in the film surface, the magnetization reversal can be achieved within a short period of time. However, since the spin transfer torque acts even after the magnetization reversal of the magnetosensitive layer as long as the electric current flows, the magnetization of the magnetosensitive layer is again reversed depending upon a time of application of the electric current. Therefore, this method requires a very precise time control of electric current.
The relaxing-precessional switching method was thus proposed and is to apply an external magnetic field in a direction of an axis of hard magnetization of the magnetosensitive layer in the precessional switching method. This method does not require the precise time control of electric current as required in the precessional switching method, but requires precise control of the spin transfer torque.
The magnetic memories as described above are described, for example, in W. C. Jeong, J. H. Park, J. H. Oh, G T. Jeong, H. S. Jeong and Kinam Kim, “Highly scalable MRAM using filed assisted current induced switching,” Symposium on VLSI Technology Digest of Technical Papers, p. 184-185, 2005 and Hiroshi Morise and Shiho Nakamura “Proceedings of The 29 th Annual Conference on Magnetics in Japan,” p183, 2005.